The present invention relates to a brake booster for a motor vehicle with two booster units each of which includes a vacuum chamber and a working chamber disposed in a casing and provided with a common partition wall. In each of these booster units the pressure difference between the vacuum and the pressure, which is controllable by a valve, in the associated working chamber works on a movable wall, which separates the vacuum chamber and the working chamber, and is transmitted by mechanical elements onto a force transmission element which combines the forces proportional to the pressure difference with the force acting on the brake pedal for transmission onto the master brake cylinder.
Such brake boosters are known for motor vehicles in which an additional force is generated from the pressure difference between a vacuum and the atmospheric pressure. The brake booster comprises a vacuum casing, a movable wall arranged therein, and a control valve. The vacuum casing is of a tubular design and the two end walls serve for fastening. Usually, the brake booster will be fastened to the splash wall in the motor vehicle by means of one end wall where the control valve is provided while at the other end wall of the vacuum casing the master brake cylinder is fastened. The inside of the vacuum casing is divided by the movable wall into a vacuum chamber with a constant vacuum and into a working chamber.
The pressure in the working chamber may reach different values dependent on the control valve operated via the brake pedal. The force which will become effective on the movable wall if there is a pressure difference will be transmitted onto the master brake cylinder via a push rod axially disposed in the vacuum casing along with the pedal force acting on the push rod via the control valve.
The vacuum casing is usually of sheet steel. The pressure difference between the atmospheric pressure prevailing outside the vacuum casing and the vacuum will generate forces which will lead to a deformation of the vacuum casing. In addition to the pressure-difference-originated forces there will be those forces that will be transmitted via the push rod during brake actuation, i.e. the brake pedal force and the boosting force generated by the brake booster. The resultant force will act on the piston of the master cylinder and generate a reaction force in the master cylinder which, while opposed to the total force, will equal it.
The pedal-force-originated constituent of the reaction force will be transmitted from the master cylinder via the steel parts of the vacuum casing to the splash wall where the brake pedal is fastened. The boosting-force-originated constituent of the reaction force will stop at the vacuum casing. The consequence of the reaction force being transmitted via the vacuum casing will be that the vacuum casing will be extended axially upon actuation of the brake. Thus, the master cylinder is displaced which will require an additional lost travel of the brake pedal. Due to the transmission ratio of the brake lever, the extension of the vacuum casing will be transmitted to the brake pedal, boosted up considerably.
The German patent DE-OS No. 2,830,262, issued Jan. 1, 1980 suggests providing at least two rods connected with the end walls of the vacuum casing substantially parallel to the push rod and penetrating the movable wall. The movable wall is slidably arranged on the rods by means of slide seals.